This invention relates to a process for electrochemically producing chlorine dioxide solutions. More particularly, this invention relates to the electrochemical production of chlorine dioxide solutions from alkali metal chlorite compounds.
Chlorine dioxide has found wide use as a disinfectant in water treatment/purification, as a bleaching agent in pulp and paper production, and a number of other uses due to its high oxidizing power. There are a number of chlorine dioxide generator systems and processes available in the marketplace. Most of the very large scale generators utilize a chlorate salt, a reducing agent, and an acid in the chemical reaction for producing chlorine dioxide. Small scale capacity chlorine dioxide generator systems generally employ a chemical reaction between a chlorite salt and an acid and/or oxidizing agent, preferably in combination. Typical acids used are, for example, sulfuric or hydrochloric acid. Other systems have also used sodium hypochlorite or chlorine as the oxidizing agent in converting chlorite to chlorine dioxide. The disadvantage of the chlorine based generating systems is the handling of hazardous liquid chlorine tanks and cylinders and the excess production of chlorine or hypochlorite depending on the system operation.
The electrochemical production of chlorine dioxide has been described previously, for example, by J. O. Logan in U.S. Pat. No. 2,163,793, issued June 27, 1939. The process electrolyzes solutions of an alkali metal chlorite such as sodium chlorite containing an alkali metal chloride or alkaline earth metal chloride as an additional electrolyte for improving the conductivity of the solution. The process preferably electrolyzes concentrated chlorite solutions to produce chlorine dioxide in the anode compartment of an electrolytic cell having a porous diaphragm between the anode and cathode compartments.
British Patent Number 714,828, published Sept. 1, 1954, by Farbenfabriken Bayer, teaches a process for electrolyzing an aqueous solution containing a chlorite and a water soluble salt of an inorganic oxy-acid other than sulfuric acid. Suitable salts include sodium nitrate, sodium nitrite, sodium phosphate, sodium chlorate, sodium perchlorate, sodium carbonate, and sodium acetate.
A process for producing chlorine dioxide by the electrolysis of a chlorite in the presence of a water soluble metal sulfate is taught by M. Rempel in U.S. Pat. No. 2,717,237, issued Sept. 6, 1955.
Japanese Patent Number 1866, published Mar. 16, 1956, by S. Saito et al. (C.A. 51,6404, 1957) teaches the use of a cylindrical electrolytic cell for chlorite solutions having a porcelain separator between the anode and the cathode. Air is used to strip the ClO.sub.2 from the anolyte solution.
Japanese Patent Number 4569, published June 11, 1958, by S. Kiyohara et al (C.A. 53, 14789d, 1959) teaches the use of a pair of membrane cells, in the first of which a concentrated NaClO.sub.2 solution is electrolyzed in the anode compartment. Air is used to strip the ClO.sub.2 from the anolyt which is then fed to the cathode compartment by the second cell. NaOH, produced in the cathode compartment of the first cell, is employed as the anolyte in the second cell.
A process for producing chlorine dioxide by the electrolysis of an aqueous solution of lithium chlorite is taught in U.S. Pat. No. 3,763,006, issued Oct. 2, 1973, to M. L. Callerame. The chlorite solution is produced by the reaction of sodium chlorate and perchloric acid and a source of lithium ion such as lithium chloride. The electrolytic cell employed a semi-permeable membrane between the anode compartment and the cathode compartment.
Japanese Disclosure Number 81-158883, disclosed Dec. 7, 1981, by M. Murakami et al describes an electrolytic process for producing chlorine dioxide by admixing a chlorite solution with the catholyte solution of a diaphragm or membrane cell to maintain the pH within the range of from 4 to 7 and electrolyzing the mixture in the anode compartment. The electrolyzed solution, at a pH of 2 or less, is then fed to a stripping tank where air is introduced to recover the chlorine dioxide.
More recently, an electrolytic process for producing chlorine dioxide from sodium chlorite has been described in which the chlorite ion concentration in the electrolyte is measured in a photometric cell to provide accurately controlled chlorite ion concentrations (U.S. Pat. No. 4,542,008, issued Aug. 17, 1985, to I. A. Capuano et al).
The electrolysis of an aqueous solution of alkali metal chlorate and alkali metal chloride in a three compartment electrolyic cell is taught in U.S. Pat. No. 3,904,496, issued Sept. 9, 1975, to C. J. Harke et al. The aqueous chlorate containing solution is fed to the middle compartment which is separated from the anode compartment by an anion exchange membrane and the cathode compartment by a cation exchange membrane. Chlorate ions and chloride ions pass into the anode compartment containing hypochloric acid as the anolyte. Chlorine dioxide and chlorine are produced in the anode compartment and chloride-free alkali metal hydroxide is formed in the cathode compartment.
An additional process for generating a chlorine dioxide solution from sodium chlorite passes a near neutral chlorite solution through an ion exchange column containing a mixture of both cation and anion ion exchange resins is described in U.S. Pat. No. 3,684,437, issued Aug. 15, 1972, to J. Callerame. The patent teaches that a very low conversion to chlorine dioxide is achieved by passing a chlorite solution through a column of cation ion exchange resin in only the hydrogen form.
There is therefore a need for a process which produces chlorine-free chorine dioxide solutions in a wide range of ClO.sub.2 concentrations continuously or on demand.